test(plugin-webpack): add tests for resolveForgeConfig

[malept]

• I have read the contribution documentation for this project.
• I agree to follow the code of conduct that this project follows, as appropriate.
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

Better test coverage in preparation for trying to fix #1444.

[NaridaL]

@malept Thanks for giving this a go!

I've starting trying to work around my issue by moving all of the dependencies, except the one I want bundled, into devDependencies and overriding electronPackager.ignore. This makes sense because the dependencies really are used only during development as "sources" for webpack.

Maybe this approach should be considered as the "official" way of doing it? It would require less magic in the webpack plugin (and no additional config options) and therefore be much more intuitive and flexible.

[codecov]

Codecov Report

Merging #1447 into master will increase coverage by 3.91%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1447      +/-   ##
==========================================
+ Coverage   71.15%   75.07%   +3.91%     
==========================================
  Files          73       73              
  Lines        2115     2118       +3     
  Branches      394      394              
==========================================
+ Hits         1505     1590      +85     
+ Misses        425      406      -19     
+ Partials      185      122      -63

Impacted Files	Coverage Δ
packages/installer/darwin/src/InstallerDarwin.ts	31.57% <0%> (-1.76%)	⬇️
packages/maker/base/src/Maker.ts	72.97% <0%> (+0.75%)	⬆️
packages/api/core/src/api/import.ts	56.86% <0%> (+0.98%)	⬆️
packages/api/core/src/api/package.ts	72.72% <0%> (+1.01%)	⬆️
packages/plugin/webpack/src/WebpackPlugin.ts	35.55% <0%> (+7.22%)	⬆️
packages/api/core/src/api/install.ts	94.93% <0%> (+16.45%)	⬆️
packages/api/core/src/api/make.ts	87.64% <0%> (+17.18%)	⬆️
packages/api/core/src/api/init.ts	97.14% <0%> (+20%)	⬆️
packages/api/core/src/api/start.ts	87.01% <0%> (+20.77%)	⬆️
packages/api/core/src/api/publish.ts	92.2% <0%> (+22.07%)	⬆️

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